Robert,

One thing that would do that is ignition timing.  When ignition timing retards, the TIT will increase.  If one ignition is dropping out under load, that will effectively retard the combustion and have the same effect.  Run a mag check at cruise and while LOP.  Make sure all EGT’s climb slightly.  If that is checks OK, I would have the timing checked on both systems.  In my opinion one gallon per hour is more than a change in air temp.  Also, the air temp in November should be cooler than it was in July.  That “should” have the effect of requiring more fuel and the plane going faster, not what you have described.

Or it could be like my plane….it won’t be happy until you spend some money on it.  For reference, When I lean to 75 degF LOP on my TNIO550 it is….1550 TIT, 16.4 to 17.0 gph, 30 inHg and 2500 RPM.  This is in a Bonanza with non-crossflow heads.  This engine has 8.5 to 1 compression ratio, so the max TIT will be lower than a stock TSIO550.

Craig Berland

I am running a TSIO550-EXP with one Lightspeed Plasma III and one mag.
For a year I would climb to cruise altitude, pull back to 32 inches and
2450 rpm, lean to 17.3 gph, and go merrily on my way. EGTs were
1550-ish, TITs were 1580-1620, and CHT varied with the season (390 in
the summer and 350 in cooler times). However about 4 months ago my EGTs
began to increase and with them the TITs. Now to keep TIT below 1625 I
must lean back to 16.3 - 16.5 gph, and that yields a slower TAS.

Anyone suggest a cause and cure?  Robert M. Simon
ES-P N301ES